Incorporation of cobaltocene as an n-dopant in organic molecular films

Electrical or chemical doping of molecular films is an efficient means of improving and controlling charge injection and carrier transport in organic devices. Recent work demonstrated that bis(cyclopentadienyl)cobalt(II) (cobaltocene, CoCp2) efficiently dopes a tris(thieno)hexaazatriphenylene (THAP) derivative, as shown by a 0.56eV shift of the Fermi level toward the empty states and an increase of current density by a factor of 103 over undoped THAP devices. In this work, a combination of x-ray photoemission spectroscopy and Rutherford backscattering is used to elucidate the details of dopant incorporation into bulk films. Cobaltocene is observed to codeposit into the THAP matrix in a controllable manner, with preferential adsorption of the dopant onto the surface of the host film. In the case of CoCp2-doped tris(8-hydroxy-quinolinato) aluminum (Alq3) films, negligible amounts of the dopant are found in the bulk matrix and on the film surface, resulting in minimal improvements in the electrical characteristics of doped Alq3 films. The process of CoCp2 adsorption onto a film surface or the evolving surface of a growing film which leads to dopant incorporation is likely assisted by charge transfer from cobaltocene to the host material, resulting in ion pairing between the dopant and host.